1. Field of the Invention
The invention relates in general to an accessing method for a memory, and more particularly to an accessing method for a memory with multi-bit cells.
2. Description of the Related Art
Along with increasing development of technology, non-volatile memory, such as flash memory, has been wildly used in electronic product. For example, a flash memory includes a number of memory cells, each of which stores a bit of data. Each of the memory cells corresponds with a programmable threshold voltage indicating a logic value of the bit of data.
Referring to FIG. 1, an illustration for threshold voltages distribution of a conventionally flash memory is shown. For example, a reference voltage PV is applied as a judging criterion. Bits of data corresponding to threshold voltages smaller than the reference voltage PV, as shown as a group of bits of data A, are programmed with a low-threshold-voltage state. It is determined that the bits of data in group A indicate logic value “1”. Bits of data corresponding to threshold voltages greater than the reference voltage PV, as shown as a group of bits of data B, are programmed with a high-threshold-voltage state. It is determined that the bits of data in group B indicate logic value “0”.
In the conventionally flash memory, the bits of data in the group B must be programmed with threshold voltages higher than the reference voltage PV and the reference voltage PV must be higher than a highest threshold voltage Vthl_h, which indicates the highest threshold voltage corresponding to the bits of data in the group A, by a margin, otherwise the conventionally flash memory cannot be properly read. Therefore, a reference voltage with high voltage level, a great number of program shots, and a great period of program time are disadvantageously needed for the conventionally flash memory.